1,720,996 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Cell-extracellular Matrix Interactions Contribute to Rapid Embryonic Wound Closure
No full textEmbryos repair wounds rapidly, in a process driven by the collective movement of the cells around the wound. The cells adjacent to embryonic wounds polarize the cytoskeletal proteins actin and non-muscle myosin II at the wound edge. Simultaneously, adherens junction proteins including E-cadherin are internalized from the wound border and localize to former tricellular junctions around the wound. Using quantitative microscopy, we found that cells adjacent to embryonic wounds in Drosophila polarized Talin, a core component of cell-extracellular matrix (ECM) adhesions. Integrin knock-down delayed wound closure and was associated with a defect in E-cadherin reinforcement at tricellular junctions along the wound edge. Additionally, depleting integrins caused a reduction in actin levels and an abnormal actin distribution around the wound. Together, our results show that cell-ECM adhesion contributes to embryonic wound repair and reveal an interplay between cell-cell and cell-ECM adhesion for the transmission of forces that drive embryonic wound healing.M.A.S
The Role of Cell Mechanics in Embryonic Wound Repair: Staggered Contraction at the Leading Edge
No full textEpithelia are physical barriers against pathogens. Therefore, the ability of multicellular organisms to self-repair epithelial wounds is critical for survival. In embryos, wound repair is mediated by the assembly of a contractile supracellular cable at the wound margin composed of filamentous actin and the molecular motor non-muscle myosin II. It has been proposed that the contraction of the actomyosin cable acts as a "purse-string" to coordinate the movement of cells into the damaged area. Here, I analyze the physical basis of the "purse string" in Drosophila embryos. Using quantitative image analysis I found that, opposing the idea of a uniform "purse string", the distribution of cytoskeletal molecules at the wound margin is heterogeneous with areas of high and low protein density. Furthermore, I showed that mutants for the non-receptor tyrosine kinase Abelson (Abl) display a homogeneous distribution of actin at the wound margin that results in slow wound repair. To investigate the role of actomyosin heterogeneity in wound healing I used biophysical tools to quantify that forces around wounds are also heterogeneous, and patches of the wound edge with heterogeneous actomyosin levels contract faster than homogeneous patches. I developed a mathematical model of wound repair that predicted that actomyosin heterogeneity benefits wound closure if myosin dynamics are directed by tension and strain. To test this idea in vivo, I inhibited stretch-activated ion channels during wound closure, which resulted in disrupted myosin dynamics and impaired tissue repair. Together these results suggest that, instead of a "purse-string", staggered contractility regulates myosin dynamics to coordinate cell movements and to drive fast wound healing.Ph.D
Actomyosin Dynamics During Coordinated Cell Movements in Embryonic Wound Repair
Get PDFEmbryos repair epithelial wounds rapidly, in a process driven by collective cell behaviors. Upon wounding, filamentous actin and the motor protein non-muscle myosin II polarize in the cells adjacent to the wound, forming a supracellular cable around the lesion. Contraction at the wound margin coordinates cell movements and drives rapid wound closure. Using FRAP in Drosophila embryos, we found that myosin turnover at the wound margin was slower than in actomyosin networks with reduced contractility. Using laser ablation we showed that tension in the wound margin cable increased as wound closure progressed, which was associated with reduced myosin turnover. Reducing tension resulted in increased turnover and loss of myosin from the wound edge. Finally, myosin motor activity was necessary for its stabilization around the wound and for efficient wound closure. Our results indicate that mechanical forces regulate myosin dynamics during embryonic wound repair, however we could not discount regulation of myosin through its binding partner, actin. To investigate a role for tension in regulating actin during wound repair, we used FRAP to show that actin was stabilized around embryonic wounds. Loss of tension through laser ablation led to loss of actin fluorescence, however, photobleaching experiments after laser severing showed no change in actin turnover. We found no changes in actin dynamics between early and late wound closure, when tension is higher. Together these data suggest that tension may be partially necessary for actin localization, but it is neither necessary nor sufficient to regulate actin turnover. To begin to pick apart the relationship between actin and myosin at the wound margin, we inhibited myosin activity and measured actin turnover. Actin stabilization was independent of myosin activity, suggesting actin may be regulated independently. To understand whether myosin turnover is independent of actin, we stabilized actin networks pharmacologically and monitored for changes in myosin dynamics. Myosin turnover was unaffected by actin stabilization. These results suggest that actin and myosin are independently regulated, and that these networks are more complex than expected. This work provides insights into the mechanisms used by cells to coordinate their behaviour and could have implications for development and disease.Ph.D.2020-01-21 00:00:0
ReSCU-Nets: recurrent U-Nets for segmentation of multidimensional microscopy data
Get PDFSegmenting multi-dimensional microscopy data requires high accuracy across many images (e.g.timepoints or Z slices) and is thus a labour-intensive part of biological image processing
pipelines. We present ReSCU-Nets, recurrent convolutional neural networks that use the
segmentation results from the previous frame as a prompt to segment the current frame. We
demonstrate that ReSCU-Nets outperform state-of-the-art image segmentation models, including
nnU-Net and the Segment Anything Model, in different segmentation tasks on time-lapse
microscopy sequences. Using ReSCU-Nets, we investigate the role of gap junctions during
Drosophila embryonic wound healing. We show that pharmacological blocking of gap junctions
slows down wound closure by disrupting cytoskeletal polarity and cell shape changes necessary
to repair the wound. Our results demonstrate that ReSCU-Nets enable the analysis of the
molecular and cellular dynamics of tissue morphogenesis from multidimensional microscopy
data.M.A.S
Automated Cell Tracking Identifies Mechanically-Oriented Cell Divisions During Drosophila Axis Elongation
Get PDFAxis elongation is a conserved process in which embryos establish their anterior-posterior (AP) axis. Drosophila axis elongation occurs in an epithelial monolayer, the germband, and is driven by cell intercalation, cell shape changes and oriented cell divisions at the posterior end of the tissue. We found that anterior germband cells divided during axis elongation. We developed automated image analysis and machine learning algorithms to investigate anterior divisions. In the anterior germband, mesectoderm cells, which form the ventral midline, divided parallel to the AP axis, while lateral cells displayed a uniform distribution of division orientations. Mesectoderm cells did not intercalate and exhibited greater AP strain before division. Using laser ablation, we isolated mesectoderm cells from anterior-posterior or dorsal-ventral forces, and we found that oriented divisions required AP-oriented forces. Our data suggest that AP-oriented forces deform germband cells, and mesectoderm cells alleviate their deformation by dividing along the axis of greater strain.M.A.S
Cellular Mechanisms of Embryonic Tissue Organization
No full textDuring early embryonic development, cell and tissue movements must be carefully orchestrated to prevent developmental anomalies. Here, I characterize cellular dynamics during axis elongation, a conserved developmental process in which the head-to-tail (anterior-posterior) axis of an animal extends. In Drosophila, early stages of axis elongation are driven by cell intercalation in the ectoderm, in which cells exchange neighbours through the formation of transient multicellular vertices. Vertices are resolved by the systematic assembly of new cell interfaces parallel to the head-to-tail axis of the embryo. I show that new junctions elongate in pulses anti-correlated with the periodic contractions of the cells anterior and posterior to the new contact. Inhibiting actomyosin contractility disrupts both the rate and directionality of new junction assembly. Disrupting contractility in the cells anterior and posterior to the new edge disrupts vertex resolution and slows down new edge elongation, while preventing contraction of the dorsal and ventral cells mainly affects the maintenance and lengthening of the new cell interface. Hypercontraction of the cells anterior and posterior to the new edge accelerates the rate of new edge assembly. Finally, applying ectopic tension orthogonal to the characteristic orientation of vertex resolution is sufficient to alter the direction of new edge formation, suggesting that local mechanical forces associated with actomyosin contractility direct the assembly of new cell contacts during multicellular vertex resolution during ectoderm extension. During later stages of axis elongation, a cell population on the underside of the embryo, the mesectoderm, undergoes oriented cell divisions to relieve strain generated from intercalating cells. Following mitoses, cell polarity is reversed in mesectoderm cells, resulting in the formation of supracellular myosin cables that segregate the mesectoderm from the ectoderm. Increased tension at the boundary prevents cell mixing and contributes to maintenance by stabilizing myosin. Mechanical and pharmacological cable disruption results in the premature internalization of the mesectoderm. Thus, our findings demonstrate that myosin cables can maintain tissue position while acting as boundaries that prevent cell mixing. Together, my work identifies two distinct mechanisms by which subcellular dynamics can translate into tissue-level organization during embryonic development.Ph.D
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